The present invention relates to an improved arrangement of electrolytic cells of the filter press type. More particularly, the present invention relates to a unitary arrangement of filter press cells in a circuit wherein the circuit has a common support structure positioned beneath the circuit which includes the electrolyte feed and the electrolysis products collection conduits. Electrolytic cells are particularly useful in the electrolysis of alkali metal chlorides, such as sodium choride, to produce alkali metal hydroxides, such as sodium hydroxide, together with chlorine and hydrogen.
A filter press arrangement typically consists of a plurality of separate cell units having planar electrode elements generally mounted in a vertical position separated along their active faces by a barrier, such as a diaphragm or membrane layer. The filter press cell units may be monopolar or bipolar and may be appropriately connected in series or parallel to form a cell circuit or bank.
Chlorine and alkali metal hydroxides are essential and large volume commodities as basic industrial chemicals. Plants producing 500 to 1000 tons of chlorine per day are not uncommon. Such plants typically utilize a large number of individual electrolytic cells having current capacities of several hundred thousand amperes. Thus, minor improvements in individual cell operation or performance have major economic benefits because of the volume of the products produced.
Upon the application of direct, electrolyzing current to an electrolytic cell containing an aqueous solution of an alkali metal chloride as the electrolyte, hydrogen and alkali metal hydroxide are produced at the cathode and chlorine is produced at the anode.
Electrolytic cells that are commonly employed commercially for the conversion of alkali metal halides into alkali metal hydroxides and halides may be considered to fall into the following general types: (1) diaphragm, (2) mercury and (3) membrane cells.
Diaphragm cells utilize one or more diaphragms permeably to the flow of electrolyte solution but impervious to the flow of gas bubbles. The diaphragm separates the cell into two or more compartments. Although diaphragm cells achieve relatively high product per unit floor space, at low energy requirements and at generally high current efficiency, the alkali metal hydroxide product, or cell liquor, must be concentrated and purified. Such concentration and purification is usually accomplished by a subsequent evaporation step.
Mercury cells typically utilize a moving or flowing bed of mercury as the cathode and produce an alkali metal amalgam in the mercury cathode. Halide gas is produced at the anode. The amalgam is withdrawn from the cell and treated with water to produce a high purity alkali metal hydroxide.
Membrane cells utilize one or more membranes or barriers separating the catholyte and the anolyte compartments. The membranes are permselective, that is, they are selectively permeably to either anions and cations. Generally, the permselective membranes utilized are cationically permselective. Usually, the catholyte product of the membrane cell is a relatively high purity alkali metal hydroxide ranging in concentration from about 250 to about 350 grams per liter.
The advent of dimensionally stable anodes has permitted ever narrowing of the space, or gap, between the electrodes of a cell, thereby facilitating progressively higher cell efficiency. The advent of dimensionally stable anodes and suitable membrane materials has made possible the construction of electrolytic cells having a thin separating partition positioned between planar electrodes, and the combination of a number of individual cell units, usually between about 10 and about 100, to form a cell circuit or bank arranged in the manner of a filter press. Circuits or banks of filter press cells are formed by the assembly of individual cell components. For example, in the case of a monopolar arrangement, the components typically would comprise a plurality of anodes mounted in anode frames and cathode mounted in cathode frames. The anodes and cathodes are separated along their active faces by a permeably barrier, such as a diaphragm or membrane, forming an anolyte compartment and a catholyte compartment, and along the inner periphery of their frames by a pliable or elastic gasket member. The assembly is completed by coupling or pressing the components together, hydraulically or by means of threaded connectors, to compress the gasket members to form gas and liquid-tight seals between the individual units.
In a typical electrolysis operation employing a circuit of filter press cells to electrolyze sodium chloride, an anolyte feed, an aqueous solution of brine containing between about 100 and about 310 grams per liter sodium chloride, is introduced into each of the anolyte compartments, and a catholyte, water or a recirculating solution of sodium hydroxide, is introduced into each of the catholyte compartments. When an electrolyzing source of direct current is imposed on the circuit, chlorine is formed at the anodes and is removed from the anolyte compartments through suitable vents or ports, along with a portion of the brine, and fed into a common collector conduit along with the anodic products from other cells in the circuit. Hydrogen and sodium hydroxide formed by migration of hydroxyl ions through the barrier member, are removed from the catholyte compartments through suitable vents or ports and fed into a separate common collector conduit along with the cathodic products from other cells in the circuit.
The present invention provides a means of support beneath a cell circuit which is combined with the various feed and product conduits. The present unitary circuit may be removed from service for repair by making suitable electrical connection around the circuit and disconnecting only the feed and product connections for the circuit. The circuit may then be lifted as a unit and replaced by a similar circuit.